A variety of imaging applications require powders with specific particle size distributions and electrostatic charge; and frequently, complex compositions. A few examples include dry or liquid electrophotographic toners, electrostatic charge control agents used for dry or liquid toners or powder coatings, and fluorescent or phosphorescent marking particles used for security or identification purposes. Current processes used to produce these particles are typically complex, expensive, and may not be versatile enough to produce a wide variety of such powders.
The use of dry toners to print electrostatic or magnetographic images has been practiced for over 50 years. Such toners have evolved from relatively simple compositions of polymer and carbon black pigment to today's toners which typically comprise one or more polymers, pigments, waxes, charge control agents and a wide variety of particulate additives. Imaging of such toners is most commonly accomplished by an electrophotographic process that involves charging of a photoreceptor, selective discharge of the photoreceptor via light lens, laser or LED, imagewise deposition of toner particles onto the photoreceptor, transfer of the toner particles to paper or other substrate and fusing of the deposited toner. Typically the photoreceptor is cleaned for reuse.
Current electrophotographic printing systems are placing increasing demands on toners, both in terms of image quality and also machine reliability. The toners must provide sharp, dense, low background, and well-fixed images. Color images must have appropriate hue, saturation and a certain level of gloss. The toners must be compatible with printer sub systems to contribute to photoreceptor cleaning and fuser release. To achieve these goals toners are designed with smaller and narrow particle size distributions, stable charge distributions, and specific melt rheology. These toner properties are obtained by specific compositions of polymers, pigments, colorants, charge control agents, waxes and miscellaneous additives.
Meeting the requirements of current toner-based hardware has become a challenge for toner designers. In particular, there are issues related to using significant quantities of wax in toners, uniform toner charging, ability to use novel polymers, and control of particular shape and size. Each of these issues is addressed by the current invention. In addition, the current invention allows for the production of unique toners and additives that can be incorporated into toners and powders to improve their performance.
A first aspect of the present invention provides a versatile process for preparing lower cost toner charge control agents, and particularly ones that offer benefits over current commercial materials.
Electrostatic charging of toner (usually referred to as “triboelectric” charge after the Greek word “tribo” meaning “to rub”) occurs when toner contacts a second component carrier or other charging surface. This toner charging is a surface phenomenon dependent on the physics and chemistry of both the toner and charging surface. In single component developers this charging surface can be a metering blade or charge roll, often coated with polymers or specialty chemicals that can influence toner charge. In dual component systems a carrier serves a dual function of charging and transporting toner. The carrier surface is typically coated with polymers that provide both durability and at least some degree of toner charging.
While toner charging can be influenced by the secondary charging surface, it is usually the toner composition itself that has the greatest influence on most charge properties. Toner polymers, for example, have a certain intrinsic charge that could be positive or negative, depending on their composition. Usually those binders are various styrene acrylic or polyester compositions and the vast majority have a tendency to charge negative. Inclusion of carbon black in a toner can raise this negative charge magnitude, particularly if the carbon has an acidic functionality. However, addition of carbon black will not usually provide fast charge rate, particularly when admixing new toner with existing toner. Likewise carbon black alone will not change the charge polarity. In addition, carbon black is not suitable for color toners. Another technique to influence toner charging is to attach particulate additives to the toner surface.
While the materials and techniques described above can serve to modify toner charge, they are often not sufficient to provide the complete range of charge polarity, charge magnitude, charge rate and charge stability. A solution has been to include a specialty chemical charge control agent within the toner composition. The amount of charge control agent traditionally used in these formulations may range from 0.5 to 4% or more, depending on the charge rate, charge magnitude and charge distribution desired.
The mechanism by which internal toner charge control agents work is not always well understood, and may differ depending on the charge agent composition and specific carrier or charging surface. Many of those skilled in the art have postulated electron exchange mechanisms based on “work function” differences between toner and carrier. Based on this mechanism, electron accepting functionality such as COOH, CONR2, SO3−, SO2 NR2 will predominantly tend to increase negative charging of the toner. Other researchers have postulated that ion exchange is the dominant mechanism of charge control function. Thus, substances that can release OH− or attract H+ would tend to charge positive. Proton donors or OH− acceptors would increase negative charging.
Investigations of certain ionomers in the prior art suggest the theory that mobile anions can transfer to carrier surfaces and that higher charge is obtained where one species is mobile as opposed to when both anion and cation are mobile. This theory helps explain the differing charge behavior of similar azo dyes. Many chromium based azo dyes have been claimed as negative charge control agents. However, the most effective metal complex azo dye structures have small, mobile cations such as hydrogen, sodium or ammonium. Functional groups such as nitro that help to delocalize the charge within the larger anion also appear to improve azo dye charge control function.
To be suitable as a toner charge control agent, a chemical material must meet a great many criteria. This has proven to be a difficult goal to meet as many materials will have one or more positive attributes but fail in critical areas. In general, an ideal charge control agent would possess the following characteristics. Correct charge polarity is the most obvious attribute. High charge magnitude is usually desirable as it will allow moderate concentrations of CCA to be used. High concentrations are undesirable both because of excessive expense, but also because the CCA may adversely affect toner rheology as well as its mechanical and electrical properties. Excessively high charge magnitude can also be disadvantageous as low concentrations of CCA would be required and this can be difficult to uniformly disperse. Ideally the charge magnitude will have a relatively small concentration dependence to facilitate uniform toner production. The electrostatic charge should remain stable over time, amount of mixing, and changing environmental conditions. The charge agent should provide good toner “ad-mix”, where fresh toner can be combined with existing toner and rapidly achieve the same charge. A charge agent must be thermally stable to toner processing conditions. A charge agent should be readily and uniformly dispersed within the binder resin. Poor dispersion can contribute to free charge agent particles that can contaminate photoreceptors, charge rolls, developer rolls or carrier surfaces. Additional desirable characteristics include low electrical conductivity, no interaction with machine components and non-migrating or blooming. Color toners will obviously require colorless (or the same color as the toner) charge control agents. Finally, charge agents should be of reasonable cost and be free from environmental or hazardous characteristics.
A great number of chemical materials have been patented and/or sold as a charge control agents. Among the most popular positive charge agents has been the family of aniline dyes better known as nigrosines, typified by Solvent Black 7. Nigrosine base may also be reacted with hydrochloric acid to produce an alcohol soluble variety. A sulfonate salt is Acid Black 2. Frequently the nigrosine base is reacted with stearic or oleic acid to improve polymer dispersibility. Nigrosines were among the first positive charge agents and still find extensive use today. They offer high charge, low cost and some tinting value. Disadvantages include toxicological concerns, difficulty in achieving narrow charge distributions and their dark color, which presents a problem for color toners. Other basic dyes such as triarylamines exhibit high positive charge and are more environmentally friendly, although they are also dark colored and not suitable for color toners. Colorless quaternary ammonium salts such as tetrapentyl ammonium chloride were first patented by Eastman Kodak (U.S. Pat. No. 3,893,935) during the early 70s as positive charge agents. Since that time an extensive array of colorless quaternary ammonium and phosphonium salts have been patented and used commercially. They offer advantages for color toners, are environmentally friendly, and can be relatively inexpensive. Quaternary salts are not without their disadvantages though. Electrostatic charge magnitude is not necessarily very high, heat stability can be an issue, charge rate is not always fast, and they can be more sensitive to humidity. Polymeric quaternary ammonium salts are another option for positive charge toners, but again, their charge magnitude may not be as high as desired.
It was mentioned earlier that most typical polymer/pigment combinations inherently charge negative. Thus it would not be immediately obvious that charge control agents would also be desired for negative charge toners. However, the demands of modern EP systems require fast, uniform, and stable charging. It was discovered in the mid 1970s that certain chromium complex azo dyes could contribute to rapid, stable and high magnitude electrostatic charging. The most effective of these early dyes were nitro-substituted versions with hydrogen or ammonium cations such as Acid Black 63. These dyes were highly colored and had other disadvantages, particularly the fact that they often showed a positive Ames test indicating that they may be a possible mutagen. Improved chromium complex dyes such as Hodogaya's Spilon Black TRH were later introduced (U.S. Pat. No. 4,433,040 to Niimura) and have been among the most popular current negative charge agents used by the toner industry. These dye structures have delocalized electrons and mobile cations such as hydrogen, ammonium, and sodium that contribute to higher negative charging. Although the charge characteristics of this azo dye are very effective, it is not without disadvantages that include high cost, its dark color and the fact that chromium is present. Certain iron complex azo dyes have also been found to function as toner charge control agents and these would be considered more environmentally friendly. One example is T-77, sold by Hodogaya Chemical. While this material does not contain chromium and thus is more environmentally friendly, its charge control properties are not as effective as certain chromium based charge control dyes, particularly when used with iron oxide-containing single component toners. It is not clear if this difference in performance is related to dye chemistry, degree of dispersion, or some other interaction with toner components. In U.S. Pat. No. 5,439,770 Taya teaches that using an acid functional polymer binder with an iron complex charge agent will provide improved charge properties. In U.S. Pat. No. 6,090,515 Tomiyama teaches that inclusion of a long chain alkyl compound with iron based charge agents will provide improved toners. The long chain alkyl compound has terminal —OH or —COOH groups and from 35 to 150 (—CH2-) groups. In U.S. Pat. No. 6,120,958 Ookubo teaches that a particle size of 6-15 microns is preferred for an iron based charge control agent. While these techniques can sometimes be used to improve the performance of an iron based azo dye charge agent in specific toner formulations, they are not universally acceptable in providing all of the desired toner charge characteristics with other formulations. In addition, excessive quantities of charge agent may be required and this leads to high toner cost.
For many color toners a colorless or lightly colored charge agent is required. One group of commercial colorless charge control agents comprise zinc, aluminum, or zirconium metal complexes offered by Orient Chemical or Hodogaya Chemical. Colorless polymeric charge control agents are also available from Fujikura Kasei. Environmentally friendly non-metal complexes are also available from Clariant. While these colorless or lightly colored compounds may improve the charge performance of many toner formulations, the charge rate and charge magnitude are frequently inferior to that which can be obtained by the highly colored chromium based azo dyes, and thus higher quantities may be required. These colorless compounds can also be significantly more expensive than most colored charge agents.
One option for overcoming some of the challenges of using internal charge control agents has been to use ultrafine particles on the surface of a toner as disclosed by Chatterji in U.S. Pat. No. 3,720,617. A wide variety of metal oxides, fine polymer particles, metal stearates and miscellaneous powders are commonly added to toner surfaces. These powders are usually added to improve powder flow as well as assist in photoreceptor cleaning, but they can also be used to modify toner charge. Among the most common of these additives are the silicon, aluminum and titanium metal oxides. These particles typically have an ultimate particle size of from 10 to 50 nm, with some new varieties being as large as 200 nm, however as size becomes larger it is more difficult for the particles to adhere to toner surfaces. The metal oxide particles are usually treated with silicone oil and/or silanes and titanates to control their charge and hydrophobicity.
It is possible to treat these ultrafine particles with other compounds to influence their electrostatic charge. For example, Hashimoto in U.S. Pat. No. 4,828,954 discloses treating the surface of fine particle size silica with an onium salt. Gruber in U.S. Pat. No. 4,965,158 discloses improved toners where charge enhancing additives are sorbed on the surface of flow additives. Miyakawa in U.S. Pat. No. 4,576,888 discloses a dye bonded to silica using an aminosilane coupling agent. Another patent describing ultrafine silica treated with a material to alter its charge is U.S. Pat. No. 5,178,984 where Nagatsuka treats ultrafine silica with a copolymer. Little in U.S. Pat. No. 5,900,315 produces charge-modified metal oxides by treating fine metal oxides with cyclic silazanes
In each of these patents, the silica is of a particle size significantly less than 100 nm and is intended for use as a toner surface additive, rather than be included in a toner composition as traditional charge agents are used. While externally added, treated, fine size metal oxides may improve the charging behavior of some toners, they do not satisfy all the charging requirements. Externally applied, treated metal oxides can be sensitive to high humidity with a resultant diminished print quality. The externally applied particles can also separate from toners during use and contaminate machine components as well as alter the toner charge. These ultrafine particles are also not suitable as internal charge agents because their small particle size tends to dissociate charge rather than create localized charge centers. U.S. Pat. No. 5,674,655 to Guistina discloses toner compositions where an ultrafine metal oxide is blended in a toner. However, the intended application is for odor control.
Despite the availability of numerous commercial charge control agents there continues to be a need for improved materials for both traditionally prepared extruded toners as well as new directly polymerized versions. As mentioned earlier, many typical charge agents are dyes, pigments or organic chemicals and are available as relatively large agglomerates. Toner preparation processes may break up these agglomerates to some degree but excellent dispersion is often difficult to achieve. Non-uniform dispersion results in excess charge agent in some particles and an insufficient amount in others, resulting in toners that provide non-uniform image quality and a reduced efficiency. In addition, the charge control agent can be the most expensive component of a toner and it would be desirable to be able to maintain its functional quality while reducing the quantity used. Another desired improvement would be a technique for manipulating the charge magnitude of specific charge agents without changing the total quantity of charge agent used. In this way a set of color toners could achieve similar charge characteristics using the same material. It would also be desirable for a charge agent to function both in conventional as well as direct polymerization processes. Another significant benefit of the inventive process is a technique of providing charge agents with properties tailored for a wide variety of applications, without the need of developing new chemical entities that would require separate chemical registration.
While the concept of improved charge agent particles by themselves is a suitable goal, it would be even more beneficial if additional components could be included in the charge agent composition such as wax or even specialty chemicals for security identification. In particular, the ability to include a quantity of wax in the charge agent composition would have significant benefits. From a historical perspective, the first copiers used non-contact radiant or oven fusing to soften low melt viscosity toners. A disadvantage of those systems was inadequate paper adhesion, raised images, and occasionally, paper fires. Most modern electrophotographic engines use heat/pressure rolls to soften and force the toner into paper fibers. Optimum fusing occurs when all toner adheres to the paper. A problem can occur if the complete toner layer softens but the layer splits, resulting in some “toner offset” to the hot roll fuser. To a limited extent this can be cleaned by wipers or blades. More typically hot offset toner transfer to undesired front or rear paper surfaces. One option to reduce hot offset has been to equip fuser rolls with a silicone oil lubrication system. Such systems can be messy and complex and are today commonly used only in some color and high speed printers. A novel solution to this issue was the use of fuser rolls with silicone or fluorocarbon release surfaces in combination with high cohesive strength polymers. Additionally, sharp melting point waxes are included in the toner and when melted act as an internal lubricant. Although this solution is usually quite adequate for producing a wide variety of black toners it presents challenges for color toners. Color toners are typically prepared using low melt viscosity polymer binders and these provide poor release of the toner from non-lubricated fuser rolls. Many full color systems have resorted to use of high concentrations of internal wax lubricants or release agents in a toner. However these lubricants are not usually compatible with toner polymers and they reduce shear in melt extrusion, thus making toner preparation difficult. An additional problem of toners containing melt mixed wax lubricants is that the incompatible wax may separate from the toner during milling operations. The wax particles form satellites that can adhere to toner surfaces. During the electrophotographic process these wax particles may separate from the toner and adhere to such machine components as charge rolls or photoreceptors, with resulting degradation of print image quality. Polymer/wax compatibilizers are sometimes included in a toner composition to minimize this problem but this does not provide a completely satisfactory solution.
The present invention includes a process of impregnating inorganic cores as a means of preparing novel and improved electrostatic charge control agents. This general concept of using inorganic particles as carriers for other materials is not novel in itself, and the use of relatively large size precipitated silica or inorganic particles as carriers of various liquid or polymeric compounds is well known. For example the H.M. Huber web site mentions that various inert powders such as silica, calcium silicate and calcium carbonate can function as excipients for fast-dissolving oral dosage tablets. Zucker in U.S. Pat. No. 4,344,858 discloses treated silica particles to serve as anti-foaming agents. Krivak in U.S. Pat. No. 4,717,561 discloses 0.14 to 0.84 mm silica as a carrier of vitamins. Meier in U.S. Pat. No. 5,321,070 discloses silica treated with resorcinol compounds as rubber adhesion promoters. Durand in U.S. Pat. No. 4,298,472 impregnates anhydrous silica with polar organic compounds to modify its adsorbent properties. Hench in U.S. Pat. No. 5,356,667 utilizes a porous silica to adsorb a laser dye. Xu in U.S. Pat. No. 5,555,813 uses molecular sieves to function as a carrier of a dye. Hi-Sil 223 silica, commercially available from PPG Industries, is used to absorb hexamethoxymethyl melamine resin as an adhesion promoter for wire. U.S. Pat. No. 9,085,668 discloses impregnating calcium carbonate with a fatty acid to reduce dusting and improve dispersion in a polymer system. There are also numerous patents describing treated silica for use as polymerization catalysts. It is also known to treat relatively large size pigment particles to improve their dispersion or electrostatic charge properties. U.S. Pat. No. 5,401,313 to Supplee discloses techniques to treat iron oxide particles with surface inorganic chemicals and dispersion promoting agents to improve their electrostatic charge and dispersion in such products as concrete or magnetic toners.
While these treated pigments may be suitable for their intended purpose, they would not function as toner charge control agents and they would not satisfy the main advantages provided by the inventive novel composite charge control agents. First, they would not allow for colorless charge agents and color toners. Second, the use of inorganic treatments on existing charge control agents would destroy their function. Third, the inorganic surface treatments would not provide the flexibility to use almost any existing commercial charge control agent. Fourth, the surface treatment of inorganic particles does not provide a means for incorporating additional toner-related components. Finally, they would not provide a means of significantly reducing the quantity and thus cost of using traditional charge control agents. U.S. Pat. No. 8,580,470 to Otsuka discloses mixing charge control agents with a white inorganic filler to influence toner charging. In this case a relatively small amount of the filler is admixed with large amounts of charge control agents. While this may provide some improvement over traditional charge agents it will not provide the flexibility and cost advantages of the inventive process.